1) Field of the Invention
The present invention relates to an array of micrometer-sized light emitting devices (i.e. of individual device diameter in the range 1-100 μm, and typically several 10's of μm), such as light emitting diodes (LEDs) or vertical cavity surface emitting lasers (VCSELs), and a method for making such an array. In particular, the present invention relates to an array of surface emitting LEDs based on the III-V semiconductor alloy materials system AlGaInN, deposited as a multi-layer epitaxial thin film structure on a sapphire, silicon carbide or GaN substrate or other suitable substrate known in the art.
2) Description of Related Art
In the ongoing drive towards miniaturisation of opto-electronic devices, there is a desire to fabricate arrays of very small closely spaced LEDs (“micro-LEDs”). However, reducing the dimensions and spacing of LEDs can cause fabrication problems. One particular problem that has been encountered is that of how to reliably make electrical contact to the LED material. For conventional, relatively large-scale devices (where the diameter is typically 100s of microns), a metal contact is usually deposited in a blanket form over the LEDs, thereby directly coating specific areas of the semiconductor device surface where p-type and n-type dopant-containing layers have been exposed for contacting. However, this is difficult at small dimensions and in array formats in which individual devices have to be separately contacted using metal lines that have to be run to such devices from remote contact pads. Such metal lines traverse the semiconductor surface between individual device elements and make contact thereto. Because the semiconductor surface is typically non-planar due to the etching that is required to electrically isolate devices and to expose n-type and p-type regions for contacting, this can be problematic. This is because the metallisation layers cannot provide satisfactory step-coverage over non-planar surfaces containing sharp edges, which can lead to difficulties with adhesion and to open-circuit cracks or breaks in the metal lines.
In order to make the fabrication of micro-LEDs somewhat more reliable various techniques have been proposed. In one such method, a sacrificial material, such as silicon dioxide, is deposited over the uneven surface between the LED mesas, so that it in-fills the regions between them. This silicon dioxide layer is etched or chemically and/or mechanically polished to expose upper surfaces of the LED material. A metal layer is then deposited over the entire surface, including the exposed areas. In this way, each of the LEDs is metallised. By infilling the gaps between the LEDs with silicon dioxide and then planarising the silicon dioxide, the surface presented during the metallisation stage is relatively flat. This means that metal can be reliably deposited over the surface of the LEDs.
While planarisation can and has been adopted for the fabrication of GaN-based LEDs, it is not without drawbacks. One problem is that voids can be created in the in-fill material that is used between the LED mesas. When the in-fill material is subsequently planarised, these voids can result in a pitted, non-uniform upper surface, which can in turn cause breaks in the metal contact. Furthermore, regardless of whether planarisation is achieved through an etch-back or chemical-mechanical polishing (CMP), damage to the epitaxial layer is inevitable. Where the top layer is p-type GaN, and a p-contact is to be formed on this, this is a particular problem. This is because p-type GaN is very sensitive to damage. This manifests itself in the form of undesirable electrical and optical properties in the fabricated devices. Therefore, the application of planarisation to the GaN material system is far from ideal.
Further issues with forming contacts for arrays of LEDs arise when each LED has to be individually addressable, that is where each LED has to be individually switched on and off. This can be achieved by a true individual addressing scheme, where separate contacting lines run to each device. In this case, the number of lines required for individually-addressing an array of n devices scales as n-squared, which can limit scalability of array size and number of elements. An advantageous method that achieves the same end but with fewer (2n) lines and which is therefore more scalable, is the so-called matrix-addressing scheme. In this, each row and each column of device elements shares a common metal line. When a voltage is applied to a column and a row, this causes illumination of the device that is at the intersection between the row and column.
Many arrangements have been proposed for forming arrays of individually addressable LEDs. Both individual and matrix-addressing schemes for AlGaInN micro-LED devices have been reported. An example of the former is described in the article “III-nitride blue microdisplays” by H. X. Jiang et al., Applied Physics Letters, Vol. 78, No. 9, pp. 1303-1305, 26 Feb. 2001. Examples of the latter are described in the articles “A matrix addressable 1024 element blue light emitting InGaN QW diode array”, by I. Ozden et al., Physica Status Solidi (a), Volume 188, No. 1, pp. 139-142 (2001) and “Fabrication and Performance of Two-Dimensional Matrix Addressable Arrays of Integrated Vertical-Cavity Lasers and Resonant Cavity Photodetectors” by Geib et al., IEEE Journal of Selected Topics in Quantum Electronics, Vol. 8, No. 4, July/August 2002. However, all of these prior art arrangements involve processing complications and compromises in performance that are readily apparent to those skilled in the art. The complications and compromises are related to issues of (i) scalability of array size; (ii) simplicity of the metallisation and processing scheme; (iii) optical isolation between devices (“cross-talk”), and (iv) non-optimal electrical and optical performance.